It is possible to fit an LPG conversion to a Terios, as it is to most modern vehicles.
Expect to lose around 10-15% in power when compared to petrol, costs are expensive for what is very little improvement in economy, unless you do extremely high mileages it is not worth the expense as it will take years to recoup the outlay.
You will notice that large petrol engined vehicles have LPG fitted as these vehicles, usually RR v8's, Land cruisers, etc as have a surplus of power to begin with, and can see considerable improvements in running costs, subsequently, recoupong the outlay in as little as two years for a vehicle doing average mileage.
LPG tanks are usually fitted under the vehicle, or in the boot, this takes up space, or reduces off road ability, Government plans are to raise the price of LPG in future budgets to bring it nearer to petrol in price, thus negating even more, the cost savings.